The earth's gravitational field varies, depending upon the position at which it is measured, the density of adjacent mass, the altitude or depth of the measurement and the proximity of natural topographical features and man-made artifacts of substantial mass, such as mountains and large buildings. Variations in gravitational field have been measured with precision over a substantial portion of the earth's surface, and maps have been prepared relating gravitational field intensity to position. While these maps are of scientific interest, they also have military applications. Using these maps, for example, a submerged submarine can navigate while on covert missions by monitoring gravity profiles along the ocean floor.
Accurate gravity measurement is also important to the petroleum industry. Precision gravity surveys of depleted oil or gas wells are sometimes made, in which variations in local gravity are monitored in the microgal or nano-g range, to develop data profiling variations in the density of the strata surrounding the well or borehole that may provide an indication of untapped pockets of gas or oil. Gravity measurements at this level of resolution require use of a very high precision gravity sensor, and great care must be taken during the measurements to achieve usable results. For example, the gravity sensor must be oriented so that its sensitive axis is vertically aligned. Deviation from vertical alignment by an angle .theta. produces an error equal to the value of the gravitational acceleration, g, multiplied by cos .theta.. Thus, a deviation from vertical, by an angle .theta. equal to 50 microradians contributes one nano-g (or about one microgal) error.
A conventional gravity measurement device of the type used in gravimetric logging of boreholes, typically includes a sensor fixed to a frame, that is mounted in a two-axis gimbal, so that a reference plane in the frame may be precisely leveled. The sensor is fixed at a precise angle of 90.degree. relative to the reference plane, enabling its sensitive axis to be vertically aligned by leveling the reference plane. Clearly, the angular relationship between the reference plane and the sensitive axis must be correct to avoid alignment errors. Both the lack of precision in properly leveling the reference plane and the difficulty in maintaining the precise 90.degree. angle between the sensitive axis of the gravity measurement device and the reference plane have significantly hindered the speed, resolution, and accuracy of prior art gravity measurement devices. In addition, the two-axis gimbal of such prior art devices has been limited to rotation about each axis through an angle of only about .+-.12.degree.. The limited angle of gimbal travel has made gravity measurement impossible for use in surveying boreholes having an inclination angle in excess of 12.degree..
Another approach for determining vertical alignment for gravity measurement uses a wheel rotating about a level axis. A plurality of accelerometers are mounted around the periphery of the wheel and are operative to produce an output signal that periodically cycles through a +1 g, 0 g, -1 g, and 0 g indication as the wheel rotates. The advantage of this approach over the two-axis gimbal device is that data produced by the rotating accelerometers may be processed to determine and cancel bias errors in the output signal. However, it is still necessary to level the axis of rotation of the wheel, so that the plane of the wheel is vertical. Furthermore, the signals output by the accelerometers must be transmitted to a processor through slip rings or other moving electrical connections, likely to introduce noise. Processing the signal output from a rotating accelerometer is also more difficult and potentially a source of error; the result is in part dependent on accurately timing the rotation of the wheel.
The limitations and problems associated with prior art gravity measurement devices described above, particularly with respect to accurately leveling the devices so that the sensitive axis of the gravity sensor is vertically aligned, have greatly inhibited widespread use of precision gravity surveys, particularly with respect to gravimetric borehole logging surveys. The present invention provides a simple, very accurate solution to this problem, which promises to greatly improve the accuracy and speed with which such measurements may be made. These and other advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiment that follows.